Floods and Flash Floods Hazards in Oklahoma

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					Floods and Flash Floods: Hazards in Oklahoma
Nationally, flooding and flash flooding is the number one killer of all storm-related natural
disasters. An average of about 140 people die each year in the United States due to flooding and
flash flooding. In Oklahoma, flash flooding is a consistent threat to life.

This document will focus on flooding caused by excessive precipitation. Flooding as a result of
large structural failures will be treated in a separate document.

Definitions: Flash Flooding versus River Flooding

Flooding occurs on continuous scales of time and area. Humans tend to group events into two
categories: flash flooding and river flooding. Flash flooding refers to events that occur during or
immediately after the life cycle of a thunderstorm. These events respond on the order of minutes
to hours to heavy precipitation. River flooding refers to the response of larger streams to
prolonged precipitation. Flooding on this scale may take days, or even weeks, to culminate.

Vulnerability to flash flooding and river flooding is a dichotomy, in that flash flooding’s primary
threat is to human life and safety, while river flooding’s primary threat is substantial economic
damage.

The size of a stream’s watershed is the dominant factor in the time scale of its response to heavy
precipitation. For example, very small creeks and branches of creeks can respond in minutes to
heavy precipitation. Larger rivers, such as the Arkansas, may take days to crest after prolonged
periods of rainfall.

Characteristic or Flash Flooding                            River Flooding
Factor
Size of waterway     Small (from unnamed creeks to          Large creeks, minor rivers, major rivers
                     larger creeks)
Area of watershed Several hundred acres to tens of          About a hundred to several thousand square
or catchment      square miles                              miles
Associated           Convective storms, often slow-         Larger-scale precipitation patterns, often
precipitation        moving or “training” thunderstorms     slow-moving fronts or remnants of tropical
                                                            storms, or both
Duration of event    Minutes to hours                       Hours to days
Time scale of        Minutes to hours                       Hours to days
response
Warning Lead         Short (no warning to several minutes) Significant (several hours to about a day)
Time
Location of          Within or very near the footprint of   Often downstream of heavy rainfall
flooding             heavy rainfall                         (increasingly so with larger watersheds)
Primary threat       Loss of life; human safety             Loss of property; economic disruption
Table A. Typical characteristics of, and factors related to, flash flooding and river flooding in Oklahoma.
Flash Flooding

Flash flooding occurs when the precipitation rate becomes so large that waterways cannot
evacuate the runoff, streams swell and flash flooding occurs. It can occur as soon as minutes
after a downpour has begun. The conditions that lead to flash flooding can happen anywhere in
Oklahoma, during any season, and at any time of day.

Flash flooding is a real and significant hazard to life in Oklahoma because of two major factors:
    1. The prevalence of convective precipitation. Thundershowers and thunderstorms can
        produce precipitation at very high rates. These events are certainly a large part of
        Oklahoma’s rainfall climate. They are especially prevalent during the warm season.
    2. Vehicular travel.

People often underestimate the power of water. This leads to many unfortunate and sometimes
tragic encounters during seemingly minor flooding events. As little as six inches of water can
cause a driver to lose control of a passenger vehicle.

From 1960-2002, there were 94 deaths due to flash flooding in Oklahoma. The vast majority of
these deaths are vehicle-related. Since 1994, all flash flood deaths in Oklahoma have been
vehicle-related.

Factor         Effect
Precipitation The most obvious contributing factor. As the rate of precipitation increases, so to does its
Rate          ability to outpace the ability of the watershed to absorb it. This is the dominant factor in
              flash flooding events, and can overwhelm any or all of the following factors.
Training      Storm cells that follow each other (much like box cars on a train) can repeatedly deposit
Echoes        large amounts of water on the same watershed, overwhelming its ability to handle runoff.
Slope of      Steeper topography (hills, canyons, etc.) will move runoff into waterways more quickly,
Watershed resulting in a quicker, flashier response to precipitation.
Shape of      Longer, narrower watersheds will tend to “meter out” runoff so that water arrives from
Watershed downshed (nearer to the mouth of the stream) areas faster than from upshed areas. In
              watersheds that are more square or circular than elongated, runoff tends to arrive in the
              main stem at the same time, intensifying the response. This factor becomes more
              significant with larger watersheds.
Saturation    Saturated or near-saturated soils can greatly reduce the rate at which water can soak into
of Soils      the ground. This can increase runoff dramatically.
Hardened      Extremely dry soils can develop a pavement or “crust” that can be resistant to infiltration.
Soils         This is especially true in areas of recent wildfire, where plant oils or resins may cause the
              soil to be even more water-resistant.
Urbanization The urban environment usually intensifies the response to heavy precipitation. The two
              dominant urban factors are: 1) increased pavement coverage, which prevents infiltration
              and dramatically increases runoff; and 2) Urban systems are designed to remove water
              from streets and byways as quickly as possible. This accelerates the natural response to
              precipitation by placing runoff in waterways much more quickly.
Low-water     The vast majority of flash-flood related deaths occur in vehicles. Many of these deaths
crossings     occur at low-water crossings where the driver is unaware of the depth of the water or the
              consequences of driving into it.
Table B. Contributing factors to flash-flood hazard and vulnerability in Oklahoma.
Figure 1. Flood and flash-flood related deaths in Oklahoma and nearby states, 1961-1999. Single
events strongly influenced totals in Colorado (Big Thompson, 1976, 140 deaths), Louisiana
(Hurricane Betsy, 1965, 50+ deaths), and Mississippi (Hurricane Camille, 1969, ~130 deaths).



                16


                14


                12


                10
   Fatalities




                8


                6


                4


                2


                0
                     1960
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Figure 2. Flood-related deaths in Oklahoma by year since 1960. The vast majority of these deaths
are flash flood deaths, and the vast majority of those are vehicle-related.
River Flooding

Flooding of larger streams and rivers typically requires many hours (a day or more) of
intermittent or continuous heavy precipitation over a larger area. Typical scenarios for river
flooding may involve a stalled or slow-moving front with persistent associated precipitation, the
remnants of a land-falling tropical storm, or interaction between these two features.

River flooding occurs when heavy runoff from several tributaries converge in a larger stream’s
main channel. The stream level rises, crests and drops over the course of hours or days. As a
general rule of thumb, the response to precipitation is slower for larger rivers. River flood
damage occurs over a wide area, sometimes several square miles.

Oklahoma’s vulnerability to river flooding changed dramatically during the last half of the 20th
Century. Several factors combined to minimize the loss of life due to river flooding:

   1. Physical Floodwater Control – Widespread damming of rivers and upstream tributaries
      has dramatically reduced the frequency and magnitude of river flooding in Oklahoma.
   2. More Accurate Forecasting – Hydrological forecasting has improved, as has the
      timeliness and availability of rainfall observations. As a result, the forecast level of larger
      streams is much more predictable. River stage forecasting has matured to levels of
      accuracy that were impossible early in the century.
   3. Longer Warning Lead-Times – Because river flooding typically occurs hours to days after
      rainfall ceases, warnings for river flooding often provide much more lead time than those
      for flash flooding.
   4. Floodplain Management – The state, through the Oklahoma Water Resources Board, has
      aggressively pursued a policy of mitigation through incremental reclamation of flood-
      prone areas. This has gradually reduced the number of residences in harm’s way.

As a result of these factors, the primary vulnerability to river flooding is economic in nature.
Most major economic damage is confined to a few very large events. For example, flooding
during the three years of 1957, 1984 and 1986 caused more damage than the remainder of the
years between 1955-1999 (see Figure 3).
Figure 3. Economic damage due to flooding in Oklahoma: 1955-1999. River flooding events in
1957, 1984 and 1986 constitute the majority of dollars during the period. From Climatological
Data National Summary, Annual Summary 27(13), 124; U.S. Army Corps of Engineers (1992)
and (1999).


Seasonal Trends in River Flooding

Spring and fall are the preferred seasons for river flooding, because these are the seasons that
provide the bulk of Oklahoma’s rainfall. However, river flooding can occur during any month on
the calendar.

Autumn months in Oklahoma provide an enhanced threat of wide-scale flooding because of the
combination of several contributing factors. Late summer and autumn are the peak of the Gulf
hurricane/tropical storm season. Large-scale weather patterns (fronts and upper-level storms) are
also much more active in the autumn than in the summer. Moisture from landfalling tropical
storms can interact with slow-moving fronts to provide heavy rains for days at a time. In
Oklahoma, these are the ingredients for river flooding. The moisture can be provided by tropical
storms in the Gulf of Mexico, or even from the remnants of Pacific tropical storms (see Table C).
Year Month         Tropical Source      Comments
                   Storm Region
1996 September     Fausto    Pacific    6+ inches rain; minor flooding along North Canadian.
1995 August        Dean      Gulf of    12-16 inches in parts of OK; interacted with weak, stalled cold
                             Mexico     front; major flooding along much of Salt Fork of the Arkansas
                                        River in Grant and Kay Counties; flooding also occurred on
                                        Cimarron, Washita and Arkansas Rivers.
1988 September     Gilbert   Gulf of    Interaction with slow-moving front; 4+ inch rains fell onto
                             Mexico     saturated soils; flooding on creeks and rivers.
1986 September- Paine        Pacific    Up to 20 inches in north-central OK; massive flooding on
     October                            Cimarron. flooding on the Arkansas River; ground was already
                                        saturated by rainfall associated with remnants of Pacific
                                        Hurricane Newton; estimated damages of $350 million; 52
                                        counties declared disaster areas.
1983 October       Tico      Pacific    Up to 17 inches rain in southwest and central OK; Red River
                                        at Burkburnett and Terral rose to highest stage in 60 years;
                                        widespread flooding of smaller rivers and creeks.
1981 October       Norma     Pacific    Up to 24 inches of rain in south-central OK (Monthly total of
                                        25.8” at Madill is greatest for any station during any month in
                                        OK history).
1961 September     Carla     Gulf of
                             Mexico
Table C. Selected Tropical-Storm-Related River Flooding in Oklahoma.
Appendix A: Oklahoma Flash-Flood Warnings and Events, 1986-2003
The following list contains the total number of flash flood warnings and verified flash flood
events for each of Oklahoma’s 77 counties. Warnings indicate the number of times that
meteorological conditions indicated an imminent threat of flash flooding. Verified events are the
number of flash flood events that were confirmed by National Weather Service (NWS) personnel
in follow-up investigations.

Discrepancies between the two sets of numbers are indicative that not all flash-flood events
receive warnings, and not all warnings have subsequent flooding. Flash flood forecasting is
difficult and limited somewhat by observational technology. Flash flood events in sparsely
populated areas are more difficult to verify than those in urban areas. During times of widespread
severe weather, flash flooding is only one of several hazards that threaten the public, and must
“compete” with violent weather for the time and attention of NWS staff.

            Warnings Verified   Grady         49       13      Nowata          31     25
County       Issued  Events     Grant         41       26      Okfuskee        20     17
Adair          33       19      Greer         23        8      Oklahoma        65     49
Alfalfa        31       22      Harmon        14        8      Okmulgee        38     32
Atoka          32       15      Harper         7        3      Osage           71     54
Beaver         11        4      Haskell       31       24      Ottawa          23     18
Beckham        18        5      Hughes        12        5      Pawnee          37     21
Blaine         21       10      Jackson       32       15      Payne           20     12
Bryan          57       26      Jefferson     29       13      Pittsburg       52     39
Caddo          40       19      Johnston      25        9      Pontotoc        17     5
Canadian       49       20      Kay           63       46      Pottawatomie    22     13
Carter         32       14      Kingfisher    45       21      Pushmataha      48     18
Cherokee       46       34      Kiowa         37       16      Roger Mills     11     4
Choctaw        38       16      Latimer       45       32      Rogers          44     38
Cimarron       9        10      Le Flore      71       36      Seminole        16     11
Cleveland      37       25      Lincoln       26       18      Sequoyah        34     20
Coal           17        5      Logan          4       21      Stephens        44     31
Comanche       44       30      Love          26        6      Texas           13     8
Cotton         30       12      Major         17       10      Tillman         30     14
Craig          27       20      Marshall      33       13      Tulsa           74     61
Creek          49       42      Mayes         26       15      Wagoner         38     27
Custer         18       10      McClain       30        8      Washington      29     27
Delaware       21       18      McCurtain     71       29      Washita         19     11
Dewey          13        6      McIntosh      28       21      Woods           14     11
Ellis          16        7      Murray        26       16      Woodward        14     8
Garfield       36       21      Muskogee      48       33      Statewide      2495   1470
Garvin         17        9      Noble         26       12